The present invention relates to a device for automatically detecting focus in a camera by detecting the contrast of the image of an object to be photographed with a self-scanning type photoelectric element. More particularly, the invention pertains to an improvement of such a device which makes it possible to detect focus even when the brightness of the object viewed changes.
It is a well-known principle that, for an optical image of an object to be photographed and formed through a lens, the difference in brightness of the image, that is, the contrast of light and shade of the image reaches its maximum value at the point of focus. This is due to the fact that the optical intensity (power spectrum) of the image with respect to each spatial frequency becomes a maximum at the point of focus. A variety of devices for detecting focus utilizing this phenomenon have been proposed in the art.
In one such device, the power spectrum distribution of the image of an object is electrically scanned with a self-scanning type photoelectric element producing a time-series signal electrically processed to dynamically detect the contrast. The self-scanning type photoelectric element includes a plurality of microphotoelectric elements and a scanning circuit. It is called a MOS-FET type or a CCD type depending on the arrangement of the scanning circuit employed, both of which are commercially available. The photoelectric element is capable of accumulating charges in correspondence with the quantity of light applied during a single scanning period. Accordingly, if the scanning rate of the photoelectric element is maintained unchanged, then the amount of charge accumulated thereby changes with the intensity of the incident light. As the scanning rate is made slower, the amount of charge accumulated is increased. Therefore, even if the intensity of light is low, the photoelectric element can provide a sufficient output.
In one example of an automatic focus detecting device utilizing a self-scanning type photoelectric element, a single edge portion having the highest contrast of the portions of the image of an object applied to an array of microphotoelectric elements is selected and the difference between the outputs of two microphotoelectric elements adjacent to or on both sides of the microphotoelectric element to which the edge portion is applied is sensed and focus is determined from the increases and decreases in the output difference. In the device, even if the contrast of an object to be photographed is constant, the difference between the outputs of the two microphotoelectric elements will vary as the brightness of the object is varied. If the intensities of lights applied to the two microphotoelectric elements are represented by E.sub.1 and E.sub.2, respectively, then the output difference .DELTA.V is: ##EQU1## where T.sub.o is a single scanning time period, S.sub.p is the photosensitivity of the photoelectric element, and C.sub.j is the junction capacitance.
If the brightness of an object to be photographed increases or decreases by a factor .alpha., then the output difference .DELTA.V' varies correspondingly (.DELTA.V'=.alpha..DELTA.V). Therefore, if the brightness of the object is changed, then it is impossible to properly detect the focus point. On the other hand, in a self-scanning type photoelectric element, the dynamic range of the element's output signal with respect to luminous flux is not as wide as desired. Therefore if the scanning rate is constant, the outputs of the microphotoelectric elements mentioned above may become saturated or may drop to a noise level with the result that sometimes it is again difficult to detect the output difference.
A technique for eliminating the above-described difficulties has been known in the art. With this technique, an average illumination detecting photoelectric element is disposed adjacent to the self-scanning type photoelectric element and the scanning rate of the self-scanning type photoelectric element is varied according to the magnitude of the output of the average illumination detecting photoelectric element. That is, an output equivalent to the average ambient illumination is provided by the single photoelectric element provided for this purpose and the output produced thereby is utilized to control the scanning rate. More specifically, if, when an average illumination E is changed into a value E' upon being increased or decreased by a factor .alpha., the outputs E.sub.1 ' and E.sub.2 ' of the two minute photoelectric elements become equal to .alpha.E.sub.1 and .alpha.E.sub.2 (E.sub.1 '=.alpha.E.sub.1, and E.sub.2 '=.alpha.E.sub.1), respectively. The output difference .DELTA.V'=.DELTA.V can be determined by setting the scanning time to T.sub.o '=T.sub.o /.alpha.. With this technique, even when the average brightness of the object is changed (either increased or decreased), the output difference of the above-described two microphotoelectric elements is maintained unchanged and the dynamic range of the photoelectric output signal with respect to the luminous flux is increased and accordingly a focus indicating signal is provided even for an object low in brightness.
However, this technique is still disadvantageous in the following point. That is, if an object moves during the measuring period or the camera is shaken, then the output of the average illumination detecting photoelectric element is varied and, accordingly, the output difference between the two microphotoelectric elements is varied.
Accordingly, an object of the present invention is to provide a focus detecting device in which the above-described difficulties have been eliminated, the contrast signal of an object to be photographed is constant even if the average brightness of the object increases or decreases, the dynamic range of the photoelectric output signal with respect to luminous flux is increased thereby to provide an effective contrast signal even for an object of low in brightness and even when an object moves or the camera is shaken so that a satisfactory focus detection is carried out.